The prior art discloses a large number of types of apparatus for measuring the quantity of flow of a liquid. One such instrument which is very popular and which has been quite successfully adapted to a large variety of uses and conditions is the so-called Rotameter which features a vertical tapered tube in which a free float is adapted to slide. The tube is tapered so that the annular space between the free float and the walls of the tapered tube becomes larger as the float is moved vertically upward within the tube. A comparatively small flow will not require so much space within which to flow as a larger, and hence the float will not be moved so far vertically upwards within the tube by a small flow as by a large. Thus the height of the float is an indication of the quantity of the flow. This type of flowmeter therefore balances the force exerted on the float by the fluid's being forced to travel in an annular path around the float against the force of gravity on the float. Since such flowmeters are ordinarily comparatively short, not more than a few feet long, the force of gravity is essentially constant over the length of the flowmeter. Therefore, the tube must be tapered in order that the net force on the float can change with its movement. If the tube were straight, the float would simply go straight to the top of the tube and remain there, thus not providing useful information.
This type of flowmeter is not without utility and, as mentioned above, has been quite widely used in the prior art. However, one difficulty with it is that it is somewhat expensive to make due to the fact that the tube must be tapered. Clearly it would be desirable to provide a flowmeter which did not require such a tapered tube to make, straight tubes being far more readily formed.